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Blurb

Plot:
The action of Opposing Force cleverly interweaves with the events of Half-Life, so that the player catches a glimpse of Gordon Freeman, the player character from the original game, sees an officer making a pivotal radio broadcast that Freeman overhears in the first game, and visits some recognizable locations.
In the original Half-Life, a "resonance cascade" opens an extra-dimensional rift, which cause Xen aliens to teleport into the Black Mesa Research Facility. The surviving scientists and security guards at the facility struggle to escape. Responding swiftly, the United States military initiates a massive "clean-up" operation to eradicate the alien presence at Black Mesa as well as "silence" any witnesses. As the game begins, Shephard is part of this operation. After his transport aircraft is shot down and he is cut off from the rest of his unit, the clean-up mission is abandoned and Shephard joins forces with the scattered survivors seeking escape.
As the tide of the furious battle begins to favor the alien army, Shephard realizes the military forces have been ordered to pull out. The Black Op forces (seen only briefly in the first game) are acting independently of the Marines, and intend to destroy the entire complex with a thermonuclear warhead. Shephard deactivates the bomb but later sees the G-Man reactivating it. After defeating a monstrous boss alien, Shephard finds himself back in a V-22 Osprey transport, similar to the one he was on at the beginning of the game, with the G-Man. The nuke at Black Mesa detonates and the G-Man detains Shephard someplace where he can do no harm and cannot be harmed, pending further evaluation...
Some interesting weapons:
Barnacle (Specimen 1176): The Black Mesa scientists were able to "tame" this Xen creature that normally adheres to ceilings. Its ability to latch onto distant organic targets (including enemies) using its long tongue makes it useful as a grappling hook allowing the player to scale heights and cross otherwise impassable gaps. Primary fire shoots out the tongue and pulls the player towards whatever it sticks to, alt-fire stops it from retracting its tongue, allowing you to swing around.
Snarks (Alternate look with HD pack) (alien weapon): Small, aggressive alien creatures that quickly pursue their target, pestering and biting, until finally exploding after several seconds (or if shot). If they cannot locate a hostile target, they will turn on the player that set them loose. When used in number, they are very effective at drawing enemies out from their cover, or distracting them from attacking the player.
Shock Roach (alien weapon): Another biological weapon, these insect-like creatures are the standard armament of the alien Shock Troopers. When a Shock Trooper dies, it drops its Shock Roach, which can only survive a few more seconds before dying itself. If the player is nearby, it will "attach" itself and allow itself to be wielded. Note that if the player already has a Shock Roach, other Shock Roaches will attack. The Shock Roach fires electric bolts that inflicts moderate damage. It can fire off ten such bolts, but continuously replenishes its "ammunition." Attempting to use the Shock Roach underwater will instantly and fatally electrocute the player.† The Shock Roach's regenerating ammunition makes it very useful as a main weapon. (†Note that firing the weapon into the water from on land does not instantly kill enemies in the water. The electricity tends to follow the path of least resistance, so whenever a charge is conducted through some substance or object, the lion's share of the current travels along the particles on the surface or skin of the conductor, given the option. Thus, shooting electricity at water will result in a lot of dispersal along the surface, and not much effect on any of the creatures inside it.)
Displacer (Project XV11382): An experimental teleportation weapon, its primary fire launches a large, slow, green orb of energy that damages enemies it passes by and obliterates anything that it hits directly. In the single-player game, the alt-fire is used to transport Shephard to Xen as well as access certain "hidden" areas of the game. In multiplayer, the alt-fire teleports the player to a random area of the map. The Displacer's primary fire is, in effect and damage, a homage to the BFG weapon of the Doom and Quake series, taking out any enemy short of a boss with a direct hit and dealing splash damage in a visible radius.